This invention relates generally to carbon nanotubes, and more particularly to methods for aligning carbon nanotubes in the production of composite materials and devices.
The exceptional mechanical properties of carbon nanotubes can be used in the development of nanotube-based, high performance structural and multifunctional nanostructural materials and devices. Carbon nanotubes are only at most several nanometers in diameter and up to several microns in length. Thus, strong interactions occur between each nanotubes due to the van der Waals forces, making good tube dispersion, desirable tube alignment and high tube loading in nanocomposites exceptionally difficult if not impossible to achieve using conventional manufacturing methods.
Many applications, such as electrical conducting, thermal conducting and high performance nanocomposites, could be achieved by preforming nanotubes into a network or membrane (5-200 μm in thickness) with controlled nanostructures (dispersion, alignment and loading). These membranes would also make nanotube materials and their properties capable of transfer into a macroscale material for easy handling. These preformed nanotube networks are also called buckypapers in literature.
Buckypapers are produced by a multiple-step process of dispersing nanotubes into a suspension and filtering the produced suspension. The produced buckypapers can be easily handled similar to conventional surface veil or glass materials. However, all the existing manufacturing techniques for nanotube membranes are discontinuous processes and can only produce small quantities and very short membrane materials, which are serious barriers for future practical applications of nanotube membranes.
Current discontinuous techniques can only produce nanotube membrane materials by filtering nanotube suspension, and the dimensions are limited by the filter dimension. In these techniques, a well-dispersed nanotube suspension is first prepared by the aid of selected surfactant and sonication. Then, a filtration system with a filter membrane of 0.1˜2 μm pore size is employed to filter the prepared suspension with the aid of vacuum or pressure. During the filtration, nanotubes will deposit onto the surface of the filter membrane to form a nanotube network. After filtration, the produced nanotube film or buckypaper can be peeled off from the filter membrane. Producing large quantities of buckypapers requires frequent changing of the filters. Current processes use many filters to complete the filtration and limit manufacture of the buckypapers to piece by piece, which is time consuming, costly and also difficult to ensure consistent product quality. More importantly, due to the limitation of filter dimension, the product membranes are of a limited length (usually less than one foot). Therefore, it would be highly desirable to provide a high quality and continuous nanotube membranes, or buckypaper materials, to increase the breadth and variety of applications in which nanotube materials can be effectively used. It would be particularly desirable to be able to construct long and continuous thermal and electrical paths in large structures or devices using carbon nanotubes in a continuous nanotube membrane or buckypaper form.